Modern vehicles, such as automobiles, trucks, buses, motorcycles, and motor homes are often equipped with sophisticated controlled braking systems that provide anti-lock braking (ABS), traction control (TCS), or stability control (SCS). During controlled braking events, an automated control unit takes control of the brake system and regulates the flow and pressure of hydraulic brake fluid to the brakes in a manner that would not be achievable through manual control by the driver.
Such controlled braking systems typically include a hydraulic pump driven by an electric motor that is activated during controlled braking events, to provide a continuous flow of pressurized brake fluid for use by control valves connected to the brakes, in accordance with control signals received from the automated control unit. A typical controlled braking pump includes a pump housing, a pair of piston pump cartridges or modules mounted in the pump housing in an opposed fashion along a reciprocating axis, and an electric motor attached to the block pump housing and having a drive shaft with an eccentric connected to pistons in the pump cartridges for driving the pistons in a reciprocating motion along the axis. The pump cartridges also typically each include inlet and outlet check valves for regulating the flow of hydraulic brake fluid in and out of the cartridges.
In prior controlled braking pumps, the inlet check valve was typically a part of and moved with the piston. U.S. Pat. No. 5,823,639, to Zinnkann et al., and U.S. Pat. No. 5,199,860 to Stegmaier, disclose piston pumps for vehicle braking systems in which the inlet check valve is located inside of the piston. The inlet valves disclosed in these patents receive fluid from passages in the sidewall of a cylinder bore housing the piston. An outlet valve is located along a reciprocating axis of the piston at the end of the cylinder bore opposite the piston.
Locating the inlet valve inside of the piston, and locating the outlet valve at the end of the cylinder bore, in the manner exemplified by Zinnkmann and Stegmaier, causes the piston to be complex, having parts that are difficult and costly to manufacture and assemble. The piston must also be made long enough to accommodate the inlet valve and provide room for movement of any moving parts of the inlet valve, thereby resulting in a piston and pump that are undesirably long. This extra length also sometimes limits the stroke of the piston that can be utilized in the space allowable for the pump under the hood of a vehicle. Having the inlet valve inside of the piston can also generate undesirable noise when the pump is operating.
In another prior approach disclosed in U.S. Pat. No. 6,220,833 B1, to Lewis, an ABS pump uses a specially shaped lip seal mounted on the outside of the piston instead of an inlet valve located inside of the piston. The piston and pump of Lewis are still undesirably complex, costly, and difficult to fabricate and assemble.
What is needed, therefore, is an improved controlled braking pump that provides a solution to one or more of the problems described above.